This invention relates to squarylium compounds, and processes and intermediates for the synthesis of these compounds. More specifically, this invention relates to processes and intermediates useful for the synthesis of squarate dyes (and to such dyes themselves) in which two heterocyclic nuclei are linked to the 1 and 3-positions of a squarate ring via a single, meso sp.sup.2 hybridized carbon atom; these dyes will hereinafter be referred to as "pentamethine squarate dyes". The processes of the present invention are especially useful for the synthesis of asymmetric pentamethine squarate dyes, that is to say those in which the two heterocyclic nuclei are dissimilar.
It is known that compounds in which two heterocyclic nuclei are linked by a pentamethine chain, the three central carbon atoms of which form part of a squarate ring, are useful as dyes, especially near infra-red dyes. (The term "near infra-red" is used herein to mean electromagnetic radiation having a wavelength of about 700 to about 1200 nm.) For example, Japanese Patent Application No. 103,604/82 (Publication No. 220,143/83, published Dec. 21, 1983), discloses a broad class of bis-heterocyclic pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium or croconylium ring. The heterocyclic nuclei can be pyrylium, thiopyrylium, selenopyrylium, benzpyrylium, benzthiopyrylium, benzselenopyrylium, naphthopyrylium, naphthothiopyrylium or naphthoselenopyrylium nuclei, which can be substituted with alkyl, alkoxy, aryl or styryl groups.
Japanese Patent Application No. 60-8730 (Publication No. 167,681/86, published Jul. 29, 1986), discloses bis(4-benz[b]thiopyrylium pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium ring. The dyes are intended for use as infra-red absorbers.
U.S. Pat. No. 4,508,811, issued Apr. 2, 1985, describes an optical recording element in which the recording layer comprises a bis(2,6-dialkyl)pyrylium or -thiopyrylium squarylium salt.
Application Ser. No. 07/6;6,639, filed Nov. 21, 1990 by Stephen J. Telfer et al. and assigned to the same assignee as the present application describes 4-[[3-[(benz[b]-4H-pyran-4-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-I- ylidene]methyl]benz[b]pyrylium hydroxide inner salt dyes, wherein at least one of the benzpyrylium nuclei carries at its 2-position a substituent in which a non-aromatic carbon atom is bonded directly to the benzpyrylium nucleus, subject to the proviso that if this 2-substituent contains an aromatic nucleus, this aromatic nucleus is not conjugated with the benzpyrylium nucleus. These dyes have high absorptions in the near infra-red, and improved solubility in semi-polar solvents and plastics.
Most of these aforementioned pentamethine squarate dyes are symmetrical, that is to say the two heterocyclic nuclei are the same. Such symmetrical dyes are typically prepared by condensing two moles of the appropriate alkyl-substituted heterocyclic compound (in most cases, a salt) with squaric acid in the presence of a base.
In certain applications of pentamethine squarate dyes, it may be advantageous to use a dye which is asymmetric, that is to say a dye which contains two different heterocyclic groupings. For example, some symmetrical near infra-red pentamethine squarate dyes have significant absorption in the visible region, and this visible absorption restricts the utility of the dyes in certain applications, for example thermal imaging media. In particular, if the symmetrical dye absorbs strongly in one part of the visible spectrum but not in another, it will tend to introduce color distortion into any image created using the symmetrical dye. Although asymmetrical analogues of these infra-red pentamethine squarate dyes may have some visible absorption, this visible absorption tends to take the form of a number of separate small peaks, and is thus more spread out over a wide range of wavelengths than in the case of the symmetrical dyes. Such absorption over a range of wavelengths tends to produce lower peak absorption and less color distortion (because the dye tends to produce a grey tint) than that produced by the symmetrical dyes, and thus the asymmetric dyes may advantageously be used in applications where the visible absorption of the symmetric dyes causes problems.
Moreover, there are a number of applications where infra-red dyes are needed which absorb at specific wavelengths. For example U.S. Pat. Nos. 4,602,263 and 4,826,976 both describe thermal imaging systems for optical recording and particularly for forming color images. These patents describe a preferred form of thermal imaging medium for forming multicolor images; in this preferred imaging medium, three separate color-forming layers, capable of forming yellow, cyan and magenta dyes respectively, are superposed on top of one another. Each of the three color-forming layers has an infra-red absorber associated therewith, these absorbers absorbing at differing wavelengths, for example 760, 820 and 880 nm. This medium is imagewise exposed simultaneously to three lasers having wavelengths of 760, 820 and 880 nm. The resultant imagewise heating of the color-forming layers causes the leuco dyes to undergo color changes in the exposed areas, thereby producing a multicolored image, which needs no development. If the choice of infra-red dyes is restricted to symmetrical compounds, it may be difficult to find a dye which absorbs at the precise wavelength required, and which meets the other requirements, such as storage stability and miscibility in polymers, for use in such media. Asymmetric dyes, which allow the two groups linked to the squarylium nucleus to be varied independently, provide an extra degree of freedom which renders it easier to find a dye which absorbs at the desired wavelength and meets the other requirements for use in such media.
However, despite the potential advantages of asymmetric pentamethine squarate dyes, little research has been conducted on such dyes because of the difficulties involved in their synthesis. Although it is possible to modify the conventional alkyl-substituted heterocyclic compound/squaric acid condensation reaction to produce asymmetric pentamethine dyes by including two different heterocyclic compounds in the reaction mixture, such a modified process inevitably produces three different products (two symmetrical dyes and the desired asymmetric dye), thereby wasting at least half the starting materials (and possibly more if one heterocyclic compound is significantly more reactive than the other). Given that the costs of some symmetric pentamethine squarate dyes are high, such materials should be used judiciously and their loss minimized where possible.
Furthermore, separation of the tertiary product mixture produced is difficult, especially since, in many cases of practical importance, the two heterocyclic compounds used are chemically similar. For example, if one attempts to produce the dye of Formula A shown in FIG. 1 in which R.sup.1 and R.sup.2 are each a hydrogen atom (this dye contains one pyrylium nucleus and one selenopyrylium nucleus) simply by condensing a mixture of the two corresponding salts with squaric acid, it is extremely difficult to separate the desired asymmetric salt from the two even on a laboratory scale and conducting this separation on a commercial scale would be a practical impossibility. In some applications of infra-red dyes, the presence of even minor amounts of symmetric by-products in the desired asymmetric dye may cause significant problems. For example in thermal imaging media described in the aforementioned U.S. Pat. Nos. 4,602,263 and 4,826,976, as already noted three separate imaging layers are present having infra-red absorbers with absorptions at 760, 820 and 880 nm. Conveniently, two of these three absorbers are Dye A shown in FIG. 1 in which R.sup.1 and R.sup.2 are each a hydrogen atom and the corresponding bis-selenopyrylium dye. However, if Dye A is contaminated with even a small proportion of the corresponding bis-selenopyrylium dye, serious problems may result in such a medium, in that the bis-selenopyrylium impurity in the layer containing Dye A will absorb the "wrong" radiation, which may lead to unwanted exposure of parts of the layer containing Dye A and a reduction in sensitivity of the medium because the bis-selenopyrylium impurity will absorb a large part of the radiation intended to cause color change in a different color-forming layer.
There is thus a need for a process for the preparation of pentamethine squarate dyes which does not require the separation of mixtures of asymmetric and symmetric products, and which can avoid waste of starting materials.
Processes for the preparation of asymmetric compounds in which two different aromatic nuclei are directly bonded to a squarate ring are known. Kazmaier et al., "The Photogenerating Properties of Unsymmetrical Squaraines and Squaraine Composites", J. Imag. Sci., 32, 1-4 (1988) states that unsymmetrical squaraines can be produced by a two-step route in which the two pendent aromatic groups are attached in separate reactions, and further states that "Unsymmetrical squaraines were synthesized in a multi-step procedure featuring the preparation of 4-(4-dimethylaminophenyl)-3-hydroxycyclobutenedione". However, no further details of this procedure are given.
U.S. Pat. No. 4,751,327 and U.S. Pat. No. 4,624,904 describe unsymmetrical squaraines for use in photoconductive imaging members. Columns 8-10 of each patent describe two synthetic methods for the preparation of these squaraines, these methods involving condensation of a diacid chloride or diester of squaric acid with one mole of a first amine, to form the appropriate 4-aminophenyl squarate derivative, hydrolysis of this derivative to introduce a 2-hydroxyl group on the squarate ring, and a second condensation to introduce at the 3-position of the squarate ring a second and different 4-aminophenyl group.
U.S. Pat. No. 4,922,018 and U.S. Pat. No. 4,886,722 describe unsymmetrical squaraines and their use in photoconductive imaging members. These squaraines are prepared by condensing, for example, a 1-alkoxyaryl-2-hydroxycyclobutene-3,4-dione derivative with an N,N-dialkylaniline derivative in the presence of an aliphatic alcohol and optionally a drying reagent. The squarate derivative is formed by a 2+2 cycloaddition process involving a tetraalkoxyolefin and an alkoxyarylketene generated in situ by the reaction of an alkoxyarylacetyl chloride and a base. The conditions of this cycloaddition reaction limit the substituents which can be present on the alkoxyarylacetyl chloride. Furthermore, the syntheses of the alkoxyarylacetyl chlorides required may be difficult.
The present invention provides processes which can be used to prepare asymmetric pentamethine squarate dyes, and intermediates produced by such processes.